A plurality of communication schemes called a third generation are adopted as IMT-2000 by ITU (International Telecommunication Union) as mobile radio communication schemes typified by mobile telephones. Among them, W-CDMA (Wide band Code Division Multiple Access) was put into commercial service in Japan in 2001.
The W-CDMA scheme aims to achieve a communication rate of about 2 Mbps (megabits per second) per mobile station at the maximum. The first specification was decided and published by 3 GPP (Third-Generation Partnership Project; http://www.3gpp.org), one of the standardization groups, as Release 1999, a version standardized in 1999. Incidentally, as a detailed manual of the W-CDMA FDD schemes in general, “W-CDMA mobile communication system”, supervised by Keiji Tachikawa, Maruzen Co., Ltd. is known.
FIG. 1 is a schematic diagram showing a conventional communication system based on the W-CDMA scheme. In FIG. 1, the reference numeral 1 designates a base station (BS), 2 designates a mobile station (MS) that carries out radio communication with the base station 1, 3 designates a downlink, 3a designates a channel (dedicated channel) assigned to the mobile station individually among the downlink 3 used by the base station 1 for transmitting data to the mobile station 2, 3b designates a channel (shared channel) transmitted to a plurality of mobile stations in common among the downlink 3, and 4 designates an uplink (dedicated channel) used by the mobile station 2 for transmitting data to the base station 1.
The W-CDMA is divided into FDD (Frequency Division Duplex) that assigns different radio frequencies to downlink 3 and uplink 4, and TDD (Time Division Duplex) that utilizes the same radio frequency and separates the downlink 3 and uplink 4 on a time division basis. Here, the FDD will be described.
Next, the operation will be described.
The downlink 3a consists of a DPDCH (Dedicated Physical Data CHannel), a data channel, and a DPCCH (Dedicated Physical Control CHannel), a control channel. Both the channels are time division multiplexed and transmitted.
The downlink 3b is a CPICH (Common Pilot CHannel) for transmitting a pilot signal for the mobile station 2 to establish synchronization with the base station 1.
The downlink 3a and downlink 3b are multiplied by spreading codes different for individual transmission data to separate the channels, followed by multiplication of a base station identification code (the so-called scramble code) assigned to the base station 1 to be transmitted.
The uplink 4 consists of a DPDCH (Dedicated Physical Data CHannel), a data channel, and a DPCCH (Dedicated Physical Control CHannel), a control channel, which are transmitted after undergoing IQ multiplexing.
The uplink 4 is multiplied by spreading codes different for individual transmission data to separate the channels, followed by the IQ multiplexing, and by multiplication by a mobile station identification code (the so-called scramble code) assigned to the mobile station 2 to be transmitted.
Recently, a large volume packet data transmission method has become popular in which a transmission rate of the downlink 3 is higher than that of the uplink 4, which is typified by the utilization of the Internet. To further increase the rate of the downlink data to be transmitted from the base station 1 to the mobile station 2 in this method, HSDPA (High Speed Downlink Packet Access), in which exclusive downlink for high-speed packet transmission is to be added, has been proposed and studied (see “High Speed Downlink Packet Access: Physical Layer Aspects (Release 5)” of 3GPP specification TR25.858 v5.0.0 (2002-03)). FIG. 2 is a diagram showing a configuration of the HSDPA. In FIG. 2, the reference numeral 5 designates an exclusive downlink for the high-speed packet transmission, and 6 designates an uplink. The remaining components are the same as those of FIG. 1.
Next, the operation will be described.
The downlink 5, which is transmitted using a so-called shared channel common to a plurality of mobile stations, is divided into a HS-DSCH (High Speed-Downlink Shared CHannel), a data channel, and a HS-SCCH (High Speed-Shared Control CHannel), a control channel.
It has been decided that the HS-DSCH employs AMC (Adaptive Modulation and Coding) that can adaptively vary a modulation scheme (such as QPSK and 16 QAM) and an error-correcting coding rate in accordance with a downlink environment (quality). In addition, because of packet transmission, retransmission control (ARQ: Auto Repeat reQuest) is carried out for reception error.
Furthermore, both the channels (HS-DSCH and HS-SCCH) are subjected to channel separation and base station identification just as the other downlinks (downlinks 3a and 3b).
In addition, to add the new downlink 5, it has been studied that the mobile station 2 transmits, to the base station 1, response data (ACK/NACK) corresponding to the downlink high-speed packet data, and downlink quality information (QI: Quality Indicator). To transmit the response data, a dedicated individual control channel (uplink 6) is added as shown in FIG. 2.
As for the uplink 6, it has been studied to separate and identify the channel using a spreading code for channel separation in the same manner as the conventional uplink channel, followed by carrying out additional IQ multiplexing to the conventional uplink 4. In TR25.858, the dedicated control channel is referred to as “HS-DPCCH” (High Speed-Dedicated Physical Control CHannel).
As for the ACK/NACK, it has been studied to transmit from the mobile station 2 only when data is transmitted from the base station 1 through the downlink 5, and is not transmitted unless a packet is transmitted. As for the QI, it is studied to transmit it from the mobile station 2 to the base station 1 periodically. Accordingly, the transmissions are performed independently.
The transmission cycle and timing offset of the QI is specified by the base station 1 as parameters in advance, and their values (report cycle k, and offset) are defined in TR25.858. The values and ranges of these values, however, are provisional values for discussion, and have not yet been determined. The provisional values of the k are 0, 1, 5, 10, 20, 40, and 80, and the ranges of the offset for each k can take values of 0≦ offset≦k−1. Since the k and offset are parameters, they can be altered halfway through the communication in accordance with a variable rate of the downlink environment.
FIG. 3 is a diagram illustrating a format of the HS-DPCCH, which will be described below.
It has been studied to separate the ACK/NACK data field from the QI data field in time, and to assign the QI twice the time assigned to the ACK/NACK. The combination of the two data is specified in terms of a time unit (Subframe) of 2 ms. The Subframe is also a transmission unit of the HSDPA downlink 5.
The report cycle k and offset are represented in terms of the Subframe used as the unit.
FIG. 4 is a diagram illustrating transmission timing of the QI excerpted. FIG. 4 illustrates an example including three mobile stations (MS's) to which the report cycle k=5 is assigned and one mobile station (MS) to which k=1 is assigned. The mobile stations with k=5 are assigned different offsets (=0, 1 and 2). In contrast, the mobile station with k=1 is assigned the offset=0, which means that the transmission is carried out consecutively because the report cycle is one.
Although the report cycle k is assumed to be one of 0, 1, 5, 10, 20, 40, and 80 at the present, their evidence is not cited. It is assumed that k=0 indicates no transmission.
FIG. 5 is a conceived internal block diagram of a base station enabling the HSDPA, and FIG. 6 is a conceived internal block diagram of a mobile station enabling the HSDPA. In FIG. 5, reference numerals 200a, 200b and 200c each designate a spreader, and 201a, 201b and 201c each designate a scrambler. The reference numeral 202 designates an adder, 203 designates a (transmitting) frequency converter, 204 designates a transmitting/receiving antenna, and 205 designates an ARQ controller for carrying out AMC operation and retransmission timing control. The reference numeral 206 designates a (receiving) frequency converter, and 207 designates a descrambler. Reference numerals 208a and 208b each designate a despreader, the reference numeral 209 designates a (time) divider, 210 designates a table for selecting an MCS from the QI, and 211 designates an MCS controller. The MCS will be described later.
In FIG. 6, reference numerals 300a and 300b each designate a spreader and 301a and 301b each designate a scrambler. The reference numeral 302 designates an adder, 303 designates a (transmitting) frequency converter, 304 designates a transmitting/receiving antenna, and 305 designates a (time) combiner. The reference numeral 306 designates a (receiving) frequency converter, and 307 designates a descrambler. Reference numerals 308a, 308b and 308c each designate a despreader. The reference numeral 309 designates a QI transmission controller, 310 designates a converter, 311 designates a QI transmission timing controller, 312 designates a data decision circuit, and 313 designates an ACK/NACK transmission timing controller.
In FIGS. 5 and 6, the parameters (k and offset) for determining the QI transmission timing are assumed to be transmitted as part of the DPDCH, the conventional data channel, and informed to the mobile station. In addition, as a downlink quality evaluation method, a method is assumed of using the SN ratio of the CPICH estimated by the mobile station. This is because the CPICH is always transmitted at a constant transmit power, which enables the evaluation of the downlink quality.
Next, the transmitting operation from the base station and the receiving operation in the base station will be described.
The data of the CPICH, a shared channel, and the data of the DPDCH/DPCCH, individual channels, are spread by the individual spreaders 200a and 200b using the different channel spreading codes according to the well-known common technique, followed by being multiplied by the mobile station identification code (scramble code) at the scramblers 201a and 201b according to the well-known common technique, and are input to the adder 202.
On the other hand, the data of the HS-DSCH/HS-SCCH, the channels for the HSDPA, are supplied to the ARQ controller 205 to undergo the transmission timing control. This is because the HSDPA channel is a shared channel for transmitting the downlink to a plurality of mobile stations, and transmits packet data. The output of the ARQ controller 205 is spread by the spreader 200c according to the well-known common technique, is multiplied by the mobile station identification code at the scrambler 201c according to the well-known common technique, and is supplied to the adder 202.
The data summed up by the adder 202, the so-called baseband frequency signal, is converted to a radio frequency signal by the (transmitting) frequency converter 203 according to the well-known common technique, and is transmitted from the transmitting/receiving antenna 204 to the mobile station as the downlink.
On the other hand, the radio frequency signal received from the mobile station by the transmitting/receiving antenna 204 is converted to a baseband signal by the (receiving) frequency converter 206 according to the well-known common technique. The baseband signal is multiplied by the scramble code, the identification number of the mobile station received, at the descrambler 207 according to the well-known common technique.
The HS-DPCCH is despread by the despreader 208a according to the well-known common technique, and is extracted as the original transmission data to be divided to the ACK/NACK data and QI information data by the (time) divider 209. The ACK/NACK data, the packet response, is supplied to the ARQ controller 205 to undergo the retransmission and timing control in accordance with the response.
The QI data separated by the (time) divider 209 is converted to the MCS (Modulation & Coding Scheme) information for packet transmission corresponding to the downlink quality (QI) by the table 210. The MCS information output from the table 210 is supplied to the MCS controller 211. The MCS controller 211 supplies the ARQ controller 205 with a signal for controlling the AMC operation, thereby carrying out the AMC operation.
The DPDCH/DPCCH, the conventional uplink channels, are despread by the despreader 208b, and are restored to the original transmission data.
Next, the operation of the mobile station will be described with reference to FIG. 6.
First, the transmitting operation of the mobile station will be described, and then the receiving operation of the mobile station will be described.
The data of the DPDCH/DPCCH, the conventional channels transmitted from the mobile station, is spread by the spreader 300a using the channel separating spreading code according to the well-known common technique, is multiplied by the mobile station identification code at the scrambler 301a according to the well-known common technique, and is supplied to the adder 302.
As for the data (ACK/NACK and QI) of the HS-DPCCH, the HSDPA channel, if any transmission data is present, it is time division multiplexed by the (time) combiner 305 in accordance with the format. Then, the data is spread by the spreader 300b using the channel spreading code according to the well-known common technique, is multiplied by the mobile station identification code at the scrambler 301b according to the well-known common technique, and is supplied to the adder 302.
The adder 302 sums up the outputs of the scramblers 301a and 301b. The output of the adder 302, the so-called baseband frequency signal, is converted to the radio frequency signal by the (transmitting) frequency converter 303 according to the well-known common technique, and is transmitted from the transmitting/receiving antenna 304 to the base station via the uplink.
On the other hand, the radio frequency signal from the base station received by the transmitting/receiving antenna 304 is converted to a baseband signal by the (receiving) frequency converter 306 according to the well-known common technique. The baseband signal is multiplied by the scramble code, the identification number of the base station received, at the descrambler 307 according to the well-known common technique.
As for the DPDCH/DPCCH, which are the conventional channels, they are despread by the despreader 308a according to the well-known common technique, and are extracted as the original data. At the same time, they are supplied to the QI transmission controller 309 that extracts and holds the QI transmission parameters.
The CPICH, the shared channel, is despread by the despreader 308b according to the well-known common technique. The converter 310 calculates the SN ratio of the CPICH from the output of the despreader 308b to generate the QI information data to be transmitted. The QI information data is transmitted as the HS-DPCCH under the timing control of the QI transmission timing controller 311 according to the parameters of the QI transmission controller 309.
As an example of the correspondence between the SN ratio of the CPICH and the QI information data, the relation as shown in Table 1 is specified in a standard in advance. This make is possible for the base station and mobile station to transmit and receive AMC controlled data using only the QI data.
TABLE 1QISN ratiotransmissionmodulation scheme,transmission(dB)dataencoding ratiorate (bps)−101QPSK, 1/3 3M−52QPSK, 1/2 5M0316QAM, 1/3 7M5416QAM, 1/210M
The HS-SDCH/HS-SCCH, the channels for the HSDPA, are despread by the despreader 308c to extract the data according to the well-known common technique. The data decision circuit 312 decides the presence or absence of an error of the extracted packet data, and generates the ACK when the error is absent, and the NACK if the error is present. The ACK/NACK data undergoes the timing control by the ACK/NACK transmission timing controller 313, and is transmitted as the HS-DPCCH.
FIG. 7 is a diagram illustrating an example of the QI transmission timing of the conventional communication system.
FIG. 7 illustrates individual QI transmission states of the system including three mobile stations with k=5 and different offsets (offsets=0, 1, 2), and one mobile station with k=10 and offset=0.
The values k and offsets can vary from mobile station to mobile station because the base station notifies the mobile stations of different values depending on the changing environment and quality of the downlinks to the mobile stations.
When the mobile stations with different k are present in the system, and if the values k have a relation of a multiple such as 5 and 10, the probability of coincidence of the transmission timing increases for a combination of particular mobile stations depending on the manner of assigning the offset (in FIG. 7, the two mobile stations (MS#1 and MS#4) with the offset=0 have the coincidence).
In addition, if these mobile stations are close to each other, the interference between the mobile stations can be increased.
With the foregoing configuration, the conventional communication system has a problem of causing interference because the QI transmission cycle parameters (other than 0 or 1) can take values having the relation of a multiple.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a communication system capable of reducing the probability of transmission collision in a combination of particular mobile stations, and reducing the interference between the mobile stations in the communication system, in which the mobile stations report the downlink quality information at alterable report cycles.